Methods of Controlling Lift - off in Conductivity Invariance Phenomenon for Eddy Current Testing
2020
Previously, a conductivity invariance phenomena (CIP) has been
discovered – at a certain lift-off, the inductance change of the sensor due to
a test sample is immune to conductivity variations, i.e. the inductance –
lift-off curve passes through a common point at a certain lift-off, termed as
conductivity invariance lift-off. However, this conductivity invariance
lift-off is fixed for a particular sensor setup, which is not convenient for
various sample conditions. In this paper, we propose using two parameters in
the coil design – the horizontal and vertical distances between the transmitter
and the receiver to control the conductivity invariance lift-off. The relationship between these two parameters
and the conductivity invariance lift-off is investigated by simulation and
experiments and it has been found that there is an approximate linear
relationship between these two parameters and the conductivity invariance
lift-off. This is useful for applications where the measurements have
restrictions on lift-off, e.g. uneven coating thickness which limits the range
of the lift-off of probe during the measurements. Therefore, based on this relationship, it can
be easier to adjust the configuration of the probe for a better inspection of
the test samples.
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